1. Field of the Invention
The present invention relates to an output driver of a semiconductor memory device and a method of driving an output of a semiconductor device, and more particularly, to an output driver of an input/output (I/O) interface in a Rambus dynamic random access memory (RDRAM).
2. Description of the Related Art
The JC-16 Subcommittee under Joint Electronic Device Engineering Council (JEDEC) has approved a gunning transceiver logic (GTL) as a standard for an interface suitable for a synchronous DRAM (SDRAM). In a RDRAM having a bandwidth more than 1.6 gigabytes per second, a Rambus signaling logic (RSL) is used as an interface. In high-speed data transmission, to reduce emission of electromagnetic interference (EMI), a swing of an output voltage needs to be reduced as much as possible.
FIG. 1 is a block diagram of a memory interface of a conventional RDRAM. Referring to FIG. 1, an I/O interface includes a control master 10, RDRAMs 11, 13, 15, and 17, transmission lines 1, 3, 5, 7, and 9, an end resistor Rterm, and a terminal voltage Vterm. Each RDRAM 11, 13, 15, or 17 includes an output driver 19, a receiver 18, and an input/output terminal 16 for the receiver 18 and the output driver 19.
FIG. 2 is a schematic diagram of the open drain output driver of FIG. 1 including a current source. Referring to FIG. 2, the conventional open drain output driver 19 includes one or more current sources connected in parallel to the output terminal 16. In this case, for convenience of explanation, only the last current source will be described. The output driver 19 controls a swing of an output voltage of the output terminal 16 using the NMOS current source consisting of NMOS transistors M1 and M2. The operation of the output driver 19 in the I/O interface, in which the plurality of RDRAMs 11, 13, 15, and 17 are coupled to the transmission lines 1, 3, 5, 7, and 9, will now be described with reference to FIGS. 1 and 2. If the NMOS transistor M2 operates in a saturation region by a predetermined reference signal Vg, then current flowing through the NMOS transistor M2 as a current source maintains a constant value. Thus, a voltage Vout of the output terminal 16 has a swing by data DAT1 input to the gate of the NMOS transistor M1.
The output driver 19 drives data having a value xe2x80x9c1xe2x80x9d or data having a value xe2x80x9c0xe2x80x9d to the single-terminated transmission lines 1, 3, 5, 7, and 9, according to the data DAT1. If the output driver 19 drives the transmission lines 1, 3, 5, 7, and 9, the output voltage Vout of the output terminal 16 may be reduced compared to a normal output voltage level by overlapping previous data and current data due to propagation delay on the transmission lines. If the output voltage Vout of the output terminal 16 is reduced and the NMOS transistor M2 operates in a linear region instead of a saturation region, current flowing through the NMOS transistor M2 from the output terminal 16 is reduced.
Consequently, if the output voltage Vout of the output driver 19 decreases, the current flowing at the output terminal 16 becomes smaller. Thus, overlapping of voltages at the transmission lines 1, 3, 5, 7, and 9 of the memory interface may hamper correct data sampling.
The present invention is therefore directed to an output driver, and a method of driving an output, which substantially overcomes one or more of the problems due to the limitations and disadvantages of the related art.
To solve the above problems, it is an objective of the present invention to provide an output driver, and a method of driving an output, for supplying constant output current independently of changes in an output voltage.
Accordingly, to achieve the above objectives, the present invention provides an output driver including a current source for regulating changes in a voltage of an output terminal in response to a reference signal, and an output current compensating circuit which responds to the current source in order to supply constant current to the output terminal.
The current source includes a first metal-oxide-semiconductor (MOS) transistor having a drain coupled to the output terminal and a gate to which the reference signal is input, and a second MOS transistor having a drain coupled to a source of the first MOS transistor, a gate to which data is input, and a source coupled to a ground voltage. For example, the current source may be an open drain current source. The output current compensating circuit includes a current compensating unit coupled to the output terminal, and a sensing driving unit for sensing voltage changes of the output terminal and driving the current compensating unit.
The above objectives may also be achieved by providing a method of driving an output including providing a current to an output terminal responsive to a reference signal, so that a voltage is realized at the output terminal; monitoring the voltage at the output terminal to provide a control signal; and maintaining the current provided to the output terminal constant responsive to the control signal.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.